Method of oxygen detection



Unite Stes Patent 3,009,097 METHOD OF OXYGEN DETECTION John P. Strange,Murrysville, Pa., assigner to Mine Safety Appliances Company,Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 19, 1958, Ser.No. 716,165 8 Claims. (Cl. 324-33) This invention relates to, and hasfor its primary object the provision of, a method for detecting thepresence and measuring the concentration of a given gas or vapor as acontaminant in a pure gas or mixture of pure gases, by the use of anionization chamber, where the contaminant is electronegative and thebackground gas is nonelectronegative, as those terms are deiined below.

Molecular gases can be classiied as belonging either l) to a type calledelectronegative, in which the molecules exhibit electron ainity or anability to pick up free electrons and form negative ions analogous tosimilar tendencies of certain well known atomic species, eg., O, C1,etc.; or (2) to a type called nonelectronegative, in which the moleculesdo not exhibit electron ainity and do not form negative ions. Thisai'nity for electrons is not dependent on ionization of the gas and isnot to be confused with the aiiinity of positive ions of an ionizedmolecule for electrons. Thus, O2, HC1, SO2, C12 may be classified asexamples of electronegative molecules, since they readily attachelectrons to form negative molecular ions. On the other hand, He, Ne, A,N2 and H2, if pure, are examples of gases that do not pick up freeelectrons and are non-electronegative.

It is known that the measured ionization current of a purenon-electronegative gas (such as H2, He2, N2, etc.) consists mostly ofpositive ions and free electrons. in an ionization chamber containing astatic sample of such a gas and a fixed radiation source, the ionizationcurrent produced by an applied electrical iield is initially (i.e.immediately after the electrical field is turned on) a function of thenumber of ions formed, the rate at which they recombine, and themobility of the charged carriers. Because the electrons are free andbecause their mobility is high, the initial ionization current is high.Also, because of the high mobility `of the free electrons, they arecollected by the positive electrode much more rapidly than the positiveions are collected by the negative electrode. However, in a staticionization chamber, the equilibrium current that is established a shorttime after the electrical lield is applied is considerably lower thanthe initial current and is a function, not only of the factorsenumerated above, but also of the space charge formed by the lowmobility positive ions.

The present invention is predicated on the discovery that providing acontinuous ilow of a pure, non-electronegative gas or gases through anionization chamber will sweep the uncollected positive ions out of thechamber and eliminate Vthe space charge effect produced by such ions,thereby permitting the ionization current in the chamber to bemaintained at a high level, approaching the initial ionization currentobtained in a static chamber. If a small amount of an electronegativecontaminant `gas or ygases is added to the stream of sample gas owingthrough such a dynamic chamber, the ionization current will be markedlyreduced, because the molecules of the contaminant, which arecharacterized by their electron ainity, will remove or reduce the numberof free electrons in the mixture to form low mobility negative ions.

While the present invention is applicable to the detection of minutequantities of any electronegative gas or gases in a non-electronegativecarrier gas or gases, it will be described herein, for convenience, inconnection with the detection of oxygen in otherwise pure hydrogen.

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Suitable apparatus for practicing the method of this invention is shownin the accompanying drawings, in which:

FIG. l shows diagrammatically an ionization chamber and an electricalcircuit for measuring changes in the ionization current in accordancewith this invention;

FIG. 2 shows part of a detector system using a second ionization chamberas a compensator chamber, with the gas sample arranged for parallel flowthrough both chambers; and

FIG. 3 is similar to FIG. 2 except that the two chambers are arrangedfor seri-es ow of the sample gas.

Referring to FIG. l, the detector ionization chamber, generallydesignated by the numeral 1, includes an outer electrode 2 in the formof a cylindrical tube of electrically conductive material. To the endsof the electrode are tted closure members 3, one of which is providedwith an inlet passage 4, for introducing a gas sample, and the otherwith an outlet passage 5. A pump (not shown) can be used to draw the gassample through the chamber. An insulated well 6 is mounted on andcommunicates with the chamber and supports an inner electrode 7, whichhas an active portion 8 coaxial with the outer electrode. A suitablesource of constant alpha radiation,

f such as a small amount of radium, is distributed within the ionizationchamber, for example, in the form of a deposit 9 on the inner wall ofthe outer electrode.

The detector ionization chamber is connected in an electrical circuitthat includes the chamber and a fixed resistor 15 of substantiallyequivalent resistance as series arms of a Wheatstone bridge. Apotentiometer rheostat 16 provides the other two arms and permits zeroadjustment o-f the bridge. A suitable voltage is applied to the bridgeby a battery 17, which is connected through a switch 18 to bridgeterminals 19 and 20. Bridge balance or unbalance is measured by aconventional cathode follower electrometer circuit, generally designatedby the symbol V, having a very high impedance; and this circuit isconnected across the bridge, between bridge terminal 21 and slider 22 ofthe potentiometer rheostat. The bridge is initially balanced with purehydrogen, for example, passing through the chamber. If a small amount ofoxygen is now added thereto, there will be a sharp decrease in theionization current in the chamber, which is translated into a voltagediterence in the bridge. The resulting bridge unbalance can be measureddirectly by the meter M, or by the required adjustment of thepotentiometer rheostat slider 22 to bring the bridge in balance. Theentire system is very sensitive and gives a measurable response to evenslight traces of oxygen.

In order to neutralize background materials in the gas sample, as wellas changes in gas pressure, temperature, rate of How, and other ambientconditions, it is frequently desira'ble to provide a second ionizationchamber that will act as -a compensator. Such a chamber 25, arranged forparallel ilow of sample gas through the compensator and detectorchambers is shown in FIG. 2, in which the electrical circuit isidentical with that of FIG. l (and accordingly not shown in full),except that the compensator chamber 25 is substituted for the fixedresistor 15. The compensator chamber is designed to be dimensionally andelectrically equivalent to the detector chamber 1. Since the compensatorchamber should respond only to changes in the background material andambient conditions of the sample gas, the oxygen must be removed fromthe sample before it enters that chamber. This may be done byintroducing the sample through an inlet 26, where it is equally dividedbetween conduits 2.7 and 28, the former leading to the detector chamber1 and the latter leading to an oxygen removal element 29, which is inturn connected to the inlet 30 of the compensator chamber. The oxygenremoval element may take a number of forms well known to the art,including chemical means for removing the oxygen, means for physicallyadsorbing the oxygen, or magnetic means for separating the oxygen bytaking advantage of its paramagnetic qualities as compared to those ofhydrogen. With the bridge balanced, the addition of oxygen to the samplegas will be reflected by a reduction of the ionization current acrossthe detector chamber, as compared with that across the compensatorchamber. Similarly, an increase in the oxygen concentration will berefiected by a greater difference between the ionization currents of thetwo chamers.

In FIG. 3, the compensator and detector chambers are connected in theelectrical circuit as in FlG. 2, but the sample gas is arranged to flowfirst through the detector chamber and then through the compensatorchamber, with the oxygen removal element between the two chambers. Sucha series fiow arrangement permits the use of a smaller gas sample thendoes the parallel flow arrangement, without any reduction in the rate offlow of the sample.

It is among the advantages of this method of gas detection that it issimple to apply, that it is extremely sensitive and accurate in itsresults, and that it involves the use of relatively inexpensivematerials. lt has been used successfully to detect the presence ofoxygen in hydrogen, ethylene and methane, where the oxygen ccncentrationin the carrier gas was as low as l parts per million. While theinvention has been described herein, for convenience, with respect tothe detection of oxygen in otherwise pure hydrogen, it will beunderstood that the same method is equally applicable to the detectionand measurement of other electronegative contaminants in an otherwisepure Sample of a non-electronegative gas or gases. For example, theprocedure described above for the detection of oxygen in otherwise purehydrogen is equally applicable to the detection of oxygen in a number ofotherwise pure non-electronegative gases, such as helium, nitrogen,argon, neon, ethylene, acetylene, paraffin hydrocarbons, etc. Likewise,the presence of other electronegative gases, such as hydrogen chloride,sulfur dioxide, chlorine, carbon monoxide, etc., may be similarlydetected as a contaminant in any of the non-electronegative gasesmentioned above. This invention is also applicable to the detection of acontaminant mixture consisting of any two or more electronegative gasesin a non-electronegative carrier gas; and the carrier gas may alsoconsist of a mixture of any two or more nonelectronegative gases. In allcases in which a compensator chamber is used, it will be understood thatthe electronegative contaminant gas or gases to be detected are removedby any suitable means from the gas stream before it enters thecompensator chamber, so that only the nonelectronegative gas or gasesenter that chamber.

While it is preferable for ionization to take place in the detector andcompensator chambers in order that the electrical field across thosechambers will be immediately operative to inhibit the recombination ofions and electrons, thereby increasing sensitivity, it will beunderstood that this invention can also be practiced successfully if thegases are ionized just before entering the electrical field.

The terms electronegative gas, non-electronegative carrier gas, andcarrier gas are used in the appended claims to designate a single gas ormixture of two or more gases of the types referred to.

According to the provisions of the patent statutes, I have explained theprinciple of my invention and have illustrated and described what I nowconsider to represent its best embodiment. However, I desire to have itunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically illustrated anddescribed.

I claim:

1. The method of detecting minute quantities of an electronegative gasin a mixture of said gas and a nonelectronegative carrier gas thatincludes the following steps: exposing substantially all of the mixtureto a source of ionization to produce positive ions and free electrons,exposing the ionized mixture to an electrical field between twooppositely charged electrodes for collecting electrons at one of thoseelectrodes to produce a measurable electron current while passing themixture through the electrical field for continuously removing from thefield positive ions formed by ionization of the mixture and negativeions formed by the attachment of free electrons to molecules of theelectronegative gas, and measuring the decrease in the resultingelectron current from that produced under the same external conditionsby a moving stream of the carrier gas alone.

2. The method according to claim 1, in which the mixture is exposed tothe source of ionization while it is passing through the electricalfield.

3. The method according to claim 1, in which the gas to be detected isat least one member selected from a group of electronegative gasesconsisting of oxygen, hydrogen chloride, sulfur dioxide, carbonmonoxide, and chlorine; and in which the carrier gas is at least onemember selected from the group of non-electronegative gases consistingof hydrogen, helium, neon, argon, nitrogen, ethylene, acetylene, andparafiine hydrocarbons.

4. The method of detecting the presence and measuring the concentrationof an electronegative gas in a mixture of said gas and anon-electronegative carrier gas that includes the following steps:separating the mixture into two moving mixture streams, removing fromthe first mixture stream the constituent to be detected so as to leave aresidual stream portion consisting of the carrier gas alone, exposingseparately the residual stream portion and the second mixture stream toequal ionizing sources for ionizing the gases therein to producepositive ions and free electrons, exposing the residual stream portionand the second of the mixture streams to separate electrical fields ofsubstantially equal intensityr defined by pairs of oppositely chargedelectrodes for collecting electrons on one electrode of each pair ofelectrodes to produce measurable electron currents across each pair ofelectrodes while passing the residual stream portion and the secondmixture stream through said electrical fields for removing from eachelectrical field positive ions formed by ionization and negative ionsformed by the attachment of free electrons to molecules of theelectronegative gas, and measuring the difference in the electroncurrent produced across each pair of electrodes.

5. The method according to claim 4, in which the residual stream portionand the second mixture stream are each exposed to the ionizing sourcewhile passing through the electrical field.

6. The method according to claim 4, in which the gas to be detected isat least one member selected from a group of electronegative gasesconsisting of oxygen, hydrogen chloride, sulfur dioxide, carbonmonoxide, and chlorine; and in which the carrier gas is at least onemember selected from the group of non-electronegative gases consistingof hydrogen, helium, neon, argon, nitrogen, ethylene, acetylene, andparaffine hydrocarbons.

7. The method of detecting the presence and measuring the concentrationof an electronegative gas in a mixture of said gas and anon-electronegative carrier gas that includes the following steps:ionizing the mixture to produce positive ions and free electrons,exposing the ionized mixture to an electrical field between a first pairof oppositely charged electrodes for collecting electrons on one ofthose electrodes to produce a measurable electron current while passingthe mixture through said electrical field for removing from between theelectrodes positive ions formed by ionization and negative ions formedby attachment of electrons to molecules of the electronegative gas, thenremoving from the mixture the constituent that is to be detected so asto leave a residual portion consisting of the carrier `gas alone,ionizing the residual portion of the mixture to produce positive ionsand free electrons, exposing the ionized residual portion to anelectrical ield between a second pair of oppositely charged electrodes,producing an electrical reld of the same intensity as that produced bythe rirst pair of electrodes for collecting electrons on one of thesecond pair of electrodes to produce a second measurable electroncurrent while passing said residual portionl through said electricalIfield for removing `from bet-Ween the second pair of electrodespositive ions formed by ionization, and measuring the difference in theelectron currents between the two pairs of electrodes.

8. The method according to claim 7, in which the gas to be detected isat least one member selected from a group of electronegative gasesconsisting of oxygen, hy-

drogen chloride, sulfur dioxide, carbon monoxide, and chlorine; andin-which the carrier gas is at least one member selected from the groupof non-electronegative gases consisting of hydrogen, helium, neon,argon, nitrogen, ethylene, acetylene, and paralline hydrocarbons.

References Cited in the le of this patent UNITED STATES PATENTS2,465,377 Jaeger Mar. 29, 1949 2,497,213 Downing Feb. 14, 1950 2,739,283Roehrig Mar. 20, 1956 2,740,894 Deisler et al Apr. 3, 1956 2,742,574Weisz Apr. 17, 1956 2,761,976 Obermaier et al. Sept. 4, 1956 2,770,772Foullres et al. Nov. 13, 1956 2,820,946 Robinson lan. 21, 1958

1. THE METHOD OF DETECTING MINUTE QUANTITIES OF AN ELECTRONEGATIVE GASIN A MIXTURE OF SAID GAS AND A NONELECTRONEGATIVE CARRIER GAS THATINCLUDES THE FOLLOWING STEPS: EXPOSING SUBSTANTIALLY ALL OF THE MIXTURETO A SOURCE OF IONIZATION TO PRODUCE POSITIVE IONS AND FREE ELECTRONS,EXPOSING THE IONIZED MIXTURE TO AN ELECTRICAL FIELD BETWEEN TWOOPPOSITELY CHARGED ELECTRODES FOR COLLECTING ELECTRONS AT ONE OF THOSEELECTRODES TO PRODUCE A MEASURABLE ELECTRON CURRENT WHILE PASSING THEMIXTURE THROUGH